Alternatives examined for weed control

Brenda Carol, Contributing Writer | Mar 13, 2008

From plant-detecting sensors on a cultivator to training sheep not to eat grape leaves, only weeds — a range of alternative weed management strategies was detailed at the recent California Weed Science Society annual meeting in Monterey.

The common thread is a shrinking pool of herbicides to win the war against weeds, particularly in specialty crops where herbicide registrations can be limited.

Technology is getting interesting in the war on weeds to say the least. Precision agriculture is getting increasingly more precise. A selective cultivator from Solex Corporation is taking precision to a new level. Imagine a cultivator that can distinguish between a weed and a lettuce plant, slice into the soil within a half-inch of the lettuce plant and dice up the weed that competes with it. That’s what the manufacturers of the Solex Robocrop are claiming. Preliminary trials in the Salinas Valley look promising. Even more interesting, the product’s manufacturers claim it can thin lettuce as well.

“Robocrop has been manufactured and marketed now in Western Europe for about six years,” says Dave Fountain, general manager, Solex Corporation in Dixon, Calif. About 100 units have been sold there.

The device is now being tested in the Salinas Valley as well as other vegetable-growing areas in California. Robocrop uses computer vision to identify crop rows, as well as individual plants within the row. The computer system then synchronizes the rotation of a cyclical cultivator to hoe between plants within the row. It eliminates the need to hand hoe in transplanted crops, according to Fountain.

The camera-guided cultivator “looks” at multiple lines in a 48-inch window, ignoring rocks and other debris that could interfere with its operation. It operates at standard tractor speeds, moving quickly and easily from 6 inches to the left or right of the crop, according to Fountain.

“As the customer gets confidence in the system, he moves the cultivator closer and closer to the crop,” Fountain says. “It’s mechanically eliminating a large percentage of the weeds.”

The device has been tested on carrots where it holds the line between plants and weeds to tolerances acceptable for good weed control, according to Fountain. In Salinas, the device was tested on beds with five lines of lettuce planted 10 inches apart. It was effective, according to Fountain.

Researchers expected to get a machine especially designed for thinning to arrive this month to test in Salinas.

“The principle of the weeder is that it has a rotating shaft with a crescent shaped knife,” Fountain says. “The computer analyzes the spacing between the plants. It speeds up and slows down the shaft so that the crescent knife makes one complete revolution between each set of plants.”

The device can cope with different stages of growth, and computer filters will also allow it to work in red lettuces, according to Fountain. The real issue, of course, is cost.

“Because it’s made in England and there’s a soft dollar now, we’re thinking it’s going to run about $25,000,” Fountain says. “We hope to have it manufactured in the United States eventually so we can reduce that cost.”

The technology is still in the development phase for factors like how it operates with drip tape in the field and other production parameters.

“We feel like we can work with plants as close as 2 inches apart,” Fountain says. “We prefer to work with plants where leaves aren’t touching. It’s going to be a learning process.”

New weed control approaches are going to be imperative in coming years due to declines in herbicide options, according to Steve Fennimore, UC weed specialist in Salinas. “New product launches have slowed considerably,” he says. “It’s especially critical in specialty crops.”

That calls for “out of the box” thinking. Sometimes alternative weed control strategies are so “out of the box” they can make a sheep stop and think. At least that’s what one UC researcher is trying to do when it comes to controlling weeds in vineyards. Morgan Doran, UC Livestock and Natural Resources advisor in Napa, Solano and Yolo counties has been working on a project using sheep to control weeds.

“There are a lot of benefits in using sheep to control weeds in vineyards,” he says. “It reduces the need for herbicides, reduces fuel and tractor costs while simultaneously increasing our flexibility in dealing with vegetation management.”

There are downsides as well. Since sheep snack on just about anything, the trick is to teach sheep to eat weeds instead of grape leaves. That’s where “aversion therapy” comes into play.

In an experiment that began in 2006 at the Hopland Research and Extension Center, sheep were first trained to avoid grape leaves by administering them with a dose of lithium chloride after ingesting the leaves. The dosage gives sheep a mild stomachache, therefore training them to avoid the source of discomfort. In 2007 they were released into a vineyard and allowed to graze.

The experiment was somewhat successful, although Doran admits there are challenges to be overcome with the system. “Aversion is not an on or off switch,” he says. “Sheep need to be retrained to avoid eating grape leaves.”